1. Field of the Invention
The invention concerns a method for the thermal adhesion of parts made of acrylic plastic with parts made of engineering plastics or of parts made of acrylic plastics or engineering plastics with one another, and a composite produced by means of this method.
2. Description of the Prior Art
Processes are known in the art for the production of composites from parts made of acrylic plastic, particularly made of poly(methyl methacrylate), with parts made of engineering plastics, such as polycarbonate, polyester, polyether, polyamide, polyketone, polysulfone, or blends thereof. In addition, processes are known for the production of a composite from parts made of an engineering plastic with another engineering plastic.
Thus, such composites can be produced, for example, with the aid of an adhesive, wherein the selection of an adhesive must be dependent on the chemical structure of the pertinent plastics (regarding adhesives, see, for example, Kirk-Othmer Encyclopedia of Chemical Technology, 3rd. Ed., Vol. 1, pp. 488-510, J. Wiley, New York, 1978). Generally, variations of adhesive methods include adhesion with the aid of a solvent, with an adhesive lacquer, or with a polymerizable adhesive. A process involving a solvent or an adhesive lacquer is feasible, particularly a halogenated solvent, because of their favorable dissolution properties with various plastics. However, halogenated hydrocarbon solvents present undesirable health and environmentally relevant risks. Furthermore, when adhesives are used to adhere plastic parts cracks may form, particularly involving the adhesion of injection molded or extruded parts, which generally exhibit characteristic stresses.
The polymerizable adhesives for the adhesion of plastic parts generally contain a monomer component of the individual plastic, certain fractions of the polymer, synthesized from such monomer components and, to enhance the acceleration of the adhesion process, a polymerization initiator, such as a peroxide, photoinitiator, or redox initiator. Also, according to this method, stress cracking can appear in a plastic part, caused by a monomeric component which acts as a solvent. The curing time with a polymerizable adhesive generally is between 1 and 3 hours, which is also a disadvantage for certain applications, particularly in (continuous) manufacturing.
Furthermore, a composite of plastics can be produced by heat sealing, which takes place at temperatures above the softening point of the pertinent plastic and generally with the application of pressure (see in this regard, for example, Vieweg-Esser, Plastics Handbook, Vol. IX, xe2x80x9cPolymethacrylates,xe2x80x9d Carl Hanser, Munich, 1975).
In the described methods for heat sealing, local overheating at the sealing site, which leads to the decomposition of the plastic and thus to the generation of gaseous monomeric components, must be carefully avoided. Therefore, flat composites made of plastics have been produced in the past predominantly by adhesion.
Another possibility for the production of a flat plastic composite is heat sealing in the presence of an intermediate layer, which adheres well under heating on both sides of plastic parts to be joined. Thus, for example, in European Patent No. A 584,593, plastic composites are described in which a composite is produced from two poly(methyl methacrylate) plastic layers by heat sealing in the presence of an intermediate layer with a thickness between 0.01 mm and several millimeters and where the intermediate layer is made of a polymer essentially synthesized from methyl methacrylate. Preferably, the intermediate layer is a thin film or plate made of poly(methyl methacrylate). In this method also, occlusions of air or monomeric decomposition products occasionally occur in the composite if the prescribed temperature control is not maintained.
Perhaps the most common method for the production of flat plastic composites made of different composites is extrusion. The technique of the coextrusion of various plastics which are melted in separate extruders and brought together in a common coextrusion nozzle to form a multilayer plastic composite is, in itself, known. European Patent No. 65,619 (U.S. Pat. No. A 4,477,521) describes plastic composites consisting of core layers synthesized from polycarbonate, polystyrene, poly(vinyl chloride), polyethylene, or styrene-butadiene mixed polymers and of cover layers synthesized from homo- or mixed polymers of methyl methacrylate. An important prerequisite for the coextrusion method is that all layers must be made of thermoplastic, extrudable plastics As shown in European Patent No. A 65,619, the thickness of the individual layers of the plastic composite must be permanently controlled and readjusted under strong standard deviations. This represents another expensive process step, in addition to the already inherently, complex coextrusion technology.
It is, therefore, an object of the present invention to satisfy the long-existing need of a technically simple method for the production of a flat plastic composite in which a joining seam is as invisible as possible and the joining can be achieved without an auxiliary, such as adhesive or film, and the joining surface is attainable without visible inhomogeneities, such as streaks or occlusions of gas.
This object and other objects as will be better understood from the following description have been surprisingly achieved by the discovery that optically satisfactory composites of at least two plastic parts can be attained as by:
a method for the production of a flat composite from at least two plastic parts, a first plastic part and a second plastic part, each having a first and a second surface, wherein said first surface is an adhesion surface and wherein said first plastic part is polymer A and said second plastic part is polymer B, comprising the steps of:
(a) preheating said adhesion surfaces of said first and second plastic parts to a temperature of 10 to 100xc2x0 K. above the Vicat softening point of said polymer A and polymer B, with the proviso that said plastic parts retain their original shape during said preheating step;
(b) pressing said first and second plastic parts between a first and a second rolls, said rolls been provided with means for exerting pressing force, so that said preheated adhesion surface of said first plastic part contacts with said preheated adhesion surface of said second plastic part; and
(c) advancing said first and second plastic parts between said rolls at an advance rate of 5 to 200 mm/sec while exerting a pressing force of 10 to 500 N.